The present invention relates to a combustion plant comprising a first circuit with a gasifying reactor which is arranged to produce a combustible gas and a degassed combustible rest product from a fuel, a first combustion chamber which is arranged to enable combustion of the combustible gas while forming combustion gases, and first means which are arranged to take advantage of energy produced in the first combustion chamber. The invention also relates to a method of combusting a fuel.
There are known combustion plants comprising a gasifying reactor in which a combustible gas is produced at substoichiometric combustion of a fuel. Such plants are employed for relatively delicate fuels, such as for example coal or waste. The combustible gas is supplied to a combustion chamber in which the gas is combusted while forming hot combustion gases which are employed to drive a gas turbine. Furthermore, such plants normally comprise an exhaust gas boiler with a steam generator and a steam cycle with a steam turbine for taking advantage of the energy in the combustion gases leaving the gas turbine. In order to obtain a high efficiency or level of employment of the fuel it is important to drive the gasifying of the fuel as far as possible. Thereby, a complication is that the rest product obtained by the gasifying consists of a liquid glass-like slag with a temperature of approximately 1000.degree. C. Another complication is that, if the gasifying is driven too far, not only volatile and non-problematic gases such as H.sub.2, CO, CO.sub.2 and H.sub.2 O are obtained in the combustible gas, but also more complicated and hard treated agents such as hydrogen sulphuric compounds. This requires an extensive purifying of the combustible gas produced from particles on one hand, and from H.sub.2 S and other mercaptans on the other hand. This purifying normally takes place in several stages, where the first stage comprises a heat exchanger by means of which the gas temperature is decreased. The following stage normally implies that solid particles are removed by means of cyclones and/or ceramic filters and the subsequent stages comprise the purifying from sulphuric compounds by means of conventional wet chemical methods. This makes such a plant very extensive, space-demanding and makes the investment cost per effect unit become high. Furthermore, the important complexity implies that the reliability and applicability of the plant is unsatisfactory at a lot of occasions. Furthermore, the efficiency of such a plant becomes restricted due to the energy demanding measures required for different purifying stages, the oxygen gas needed for the gasifying and due to the rest product obtained still having a certain energy content. Accordingly it is possible to reach a total efficiency in the order of 44% by such a plant.
It is also known to combust such delicate fuels in a pressurized, fluidized bed, a so called PFBC-plant (pressurized fluidized bed combustion). The fuel is combusted in a bed of particulate, non-combustible material which is supplied with combustion air from below through nozzles in such a way that the bed becomes fluidized. The combustion gases formed by the combustion process pass a freeboard above the bed, whereinafter they are purified with reference to particles and are conducted to a gas turbine. The combustion gases drive the gas turbine, which in its turn drives an electric generator on one hand and a compressor providing the pressure vessel with compressed air on the other hand. In the fluidized bed the set of tubes of a steam boiler with a steam generator and super heating tubes is immersed. The steam boiler is connected to a steam circuit which comprises a steam turbine for taking advantage of the produced heat. In the bed the fuel is combusted at a temperature in the order of 850.degree. C. The total nominal efficiency of the plant is in the order of 44%.
It is also known to combust such delicate fuel in a pressurized, circulating bed, a so called PCFB-plant (pressurized circulating fluidized bed). The fuel is combusted in a bed of particulate, non-combustible material supplied with combustion air from below through nozzles in such a way that the bed becomes fluidized and a significant part of the bed material is transported pneumatically through the set of tubes to a steam boiler with a steam generator and superheating tubes. The steam boiler is connected to a steam circuit comprising a steam turbine for taking advantage of the heat produced. The combustion gases formed by the combustion process, and a significant part of the bed material pass the set of tubes above the bed, whereafter, in a plurality of purifying stages, they are purified from particles, and the combustion gas is conducted to a gas turbine. The combustion gases drive the gas turbine which, in its turn, drives an electric generator on one hand and a compressor which provides the pressure vessel with compressed air on the other hand. In the bed the fuel is combusted at a temperature in the order of 850.degree. C. The total nominal efficiency of the plant is in the order of 44%.
To be able to maintain the temperature up to the first turbine stage of the gas turbine at a required level it is known through SE-B-470 222 to arrange an additional combustion in the freeboard above the bed by injecting a supplementary fuel. In that way, at partial load operation, it is possible to adjust the temperature of the combustion gases to an optimum operation temperature for the subsequent gas turbine. Such a freeboard combustion goes very well with an addition of such complementary fuels as volatile oils or gases. However, it is a disadvantage to need to employ several different types of fuels for one and the same plant, as this complicates the handling and the operation of the plant.
A problem which has burdened the PFBC- as well as PCFB-technique and obstructed the obtaining of a very high efficiency is that the upper temperature limit at which a combustion of for instance coal in a fluidized bed takes place normally is approximately 850.degree. to 950.degree. C. depending on the coal quality. This implies that the driving gas for the gas turbine comprised in the PFBC- or PCFB-power plant has a temperature approximately equal to the temperature in the fluidized bed. Because the efficiency of the gas turbine cycle increases along with increased temperature of the driving gas, it is desirable to have a higher gas temperature, up to 1200-1450.degree. C., to make the efficiency of the gas turbine cycle of the plant reach an optimum level. To remedy this weakness, it has been proposed to increase the temperature of the gases leaving the PFBC-combustion chamber or the PCFB-combustion chamber by means of a topping combustion chamber in which a fuel is combusted. As the driving gases pass the topping combustion chamber the temperature can be increased before they are supplied to the gas turbine. Such a technique is known through SE-B-458 955. In this document it is further described how fuel to the topping combustion chamber is accomplished by means of a gasifying reactor, in which coal at substoichiometric conditions is gasified while producing a combustible gas which is supplied to the topping combustion chamber. The gasifying reactor shown forms an integrated part of a PFBC-combustion chamber and is thus located inside the pressure vessel that encloses the PFBC-combustion chamber. The combustible gases produced in the gasifying reactor are conducted to a topping combustion chamber located outside the pressure vessel, where they are mixed with the combustion gases from the combustion chamber and are combusted to increase the temperature of the combustion gases to the optimum level before they are conducted to the gas turbine. However, as the combustion gases leaving the combustion chamber contain a significant amount of particles of ash, this high temperature obtained in the topping combustion chamber will result in a melting of ash, causing substantial problems in the gas turbine. To make such a combustion possible, the combustion gases from the fluidized bed must thus be purified from particles by means of not yet conventional purifying methods like hot gas filters, before the increase of temperature to such an optimum gas turbine temperature may take place. However, hot gas filters are expensive and have an unsatisfactory reliability.